Characterization of water pathways in low permeable rocks at the rock matrix scalemethodological review

Résumé

This is a methodological paper related to the characterization of water pathways, at the rock matrix or intact rock scale, in crystalline rocks candidate for the final disposal of high level radioactive wastes. At that scale, water circulation in these rocks is basically conditioned by its capacity to flow through the open effective porosity. That is, through the connected pores and fissures. Accordingly, the different procedures developed for imaging and mapping those pathways are here summarized. Among them, fluorescence microscopy, confocal laser scanning microscopy, scanning electron microscopy (secondary and backscattered electrons, and energy dispersive X-ray analysis), 14C-polymethylmethacrylate impregnation,... stand out. In addition, the two main techniques for three-dimensional reconstruction of the pore space structure, X-ray computerized tomography and confocal laser scanning microscopy are also mentioned, in special the last one with different examples. The in-situ characterization of the rock porosity and the identification of paleo-water pathways, such as uranium induced fission track micro-mapping, are also described. The main geometrical petrographic parameters related to the pore space structure are summarized and examples are also included, al.Finally, examples of petrophysical profiles containing sequential information at the granulometric scale (mm-cm) are included; their objective being is to document in detail the capacity for water and radioelements to flow into the rock matrix from hydraulically active fractures in the rock mass.